A power capacitor is a circuit element that must be used in power system compensation and filtering. During compensation or filtering of a power grid, the power capacitor is connected or disconnected through a capacitor switch, or the capacitor switch and the power capacitor are always used in combination. However, the conventional power capacitor and capacitor switch are produced in two different industries, and the power sector needs to separately purchase the two products and assemble them on equipment prior to use. Because power capacitors of different sizes require different switches, matching of different power capacitors with suitable capacitor switches must be considered in use. High-order harmonics in the power grid generate resonance under specific conditions to, cause damage of the capacitor, and in order to avoid the resonance caused by the high-order harmonics, a reactor is generally connected in series in the capacitor circuit to prevent damage of the capacitor. In practice, the series-connected reactor can only solve part of the problem of high-order harmonic resonance, and cannot solve the resonance problem fundamentally, and even with the reactor, burnout of the power capacitor also occurs due to high-order resonance. The conventional capacitor switch is a contactor-type capacitor switch, and such a switch has large power consumption generally around 15 W. The conventional combination of the power capacitor and the switch is a combination of a three-phase power capacitor and a three-phase switch. When the capacitor is disconnected through the switch, the residual voltage of one phase of capacitor always reaches 1.36 times the peak supply voltage, and the capacitor must be discharged before being connected again, so that such combination of the switch and the capacitor cannot achieve rapid capacitor switching. In addition, a discharge resistor must be installed inside the conventional power capacitor as required by safety specifications, but the discharge resistor produces large power consumption. The power consumption of the power grid caused by the discharge resistor exceeds 100 million kilowatts per year (merely according to the calculation of State Grid).
The conventional power capacitor generally adopts a mechanical thermal expansion snap-type explosion-proof mechanism in order to prevent explosion due to overheating of the capacitor. Such explosion-proof mechanism starts cutting off the capacitor when the capacitor is overheated. However, the explosion-proof mechanism is unrecoverable, and the capacitor can no longer be used once the explosion-proof mechanism operates. That is, the power capacitor that should not fail fails under some unexpected overheating conditions. Besides, such mechanical explosion-proof mechanism has high requirements on sealing of the capacitor, and the thermal expansion explosion-proof mechanism does not operate upon leakage. Further, such explosion-proof mechanism has low response speed, and cannot respond quickly to protect the capacitor when resonance occurs in the capacitor circuit.
There is also an intelligent capacitor as disclosed in Chinese Patent No. ZL200620071465.7 now on the market. In this intelligent capacitor, a three-phase capacitor and a capacitor switch are simply combined, instead of designing the power capacitor and the capacitor switch as a whole product, with the mere effect that a user does not need to worry about selecting matching switches for different capacitors and the capacitor switch and the capacitor do not need to be connected during installation. However, such intelligent capacitor still cannot effectively solve the problems of protection against capacitor heating, harmonic protection, power consumption of the capacitor switch, and rapid capacitor switching.
An external temperature protection and overcurrent and overvoltage protection device used in practice can also protect the capacitor from damage when the power capacitor is overheated or subjected to an overcurrent or overvoltage. However, the external protection device is rarely used because of the cost and mainly because the external protection device cannot detect the actual operating status in a capacitor core. The external protection device makes little sense as it cannot achieve accurate and rapid protection.
Now the capacity of general three-phase power capacitors is over 30 kvar. Due to the cost of a small-capacity power capacitor and the cost of a capacitor switch, conventional reactive compensation devices using power capacitors and having power lower than 30 kvar generally cannot achieve graded compensation, while reactive compensation devices with high power cannot achieve fine compensation.
In view of the above, the power capacitor operates in a poor power grid environment due to the presence of a large amount of harmonics. Because there is no effective, reliable and low-cost way of protecting the power capacitor, the protection of the low-voltage power capacitor remains to be a tough problem.